β‑Allyl Sulfones as Addition–Fragmentation Chain Transfer Reagents: A Tool for Adjusting Thermal and Mechanical Properties of Dimethacrylate Networks

Dimethacrylates are known to have good photoreactivity, but their radical polymerization usually leads to irregular, highly cross-linked, and brittle polymer networks with broad thermal polymer phase transitions. Here, the synthesis of mono- and difunctional β-allyl sulfones is described, and those substances are introduced as potent addition–fragmentation chain transfer (AFCT) reagents leading to dimethacrylate networks with tunable properties. By controlling the content and functionality of said AFCT reagents, it is possible to achieve more homogeneous networks with a narrow glass transition and an adjustable glass transition temperature (<i>T</i>_g), rubber modulus of elasticity (<i>E</i>_r), and network density. In contrast to dimethacrylate networks containing monomethacrylates as reactive diluents, the network architecture of the β-allyl sulfone-based dimethacrylate networks is more homogeneous and the tunability of thermal and mechanical properties is much more enhanced. The reactivity and polymerization were investigated using laser flash photolysis, photo-DSC, and NMR, while DMTA and swellability tests were performed to characterize the polymer

Phenoxazine: A Privileged Scaffold for Radical-Trapping Antioxidants

Diphenylamines are widely used to protect petroleum-derived products from autoxidation. Their efficacy as radical-trapping antioxidants (RTAs) relies on a balance of fast H-atom transfer kinetics and stability to one-electron oxidation by peroxidic species. Both H-atom transfer and one-electron oxidation are enhanced by substitution with electron-donating substituents, such as the S-atom in phenothiazines, another important class of RTA. Herein we report the results of our investigations of the RTA activity of the structurally related, but essentially ignored, phenoxazines. We find that the H-atom transfer reactivity of substituted phenoxazines follows an excellent Evans–Polanyi correlation spanning <i>k</i>_inh = 4.5 × 10⁶ M^–1 s^–1 and N–H BDE = 77.4 kcal mol^–1 for 3-CN,7-NO₂-phenoxazine to <i>k</i>_inh = 6.6 × 10⁸ M^–1 s^–1 and N–H BDE = 71.8 kcal mol^–1 for 3,7-(OMe)₂-phenoxazine (37 °C). The reactivity of the latter compound is the greatest of any RTA ever reported and is likely to represent a reaction without an enthalpic barrier since log <i>A</i> for this reaction is likely ∼8.5. The very high reactivity of most of the phenoxazines studied required the determination of their kinetic parameters by inhibited autoxidations in the presence of a very strong H-bonding cosolvent (DMSO), which slowed the observed rates by up to 2 orders of magnitude by dynamically reducing the equilibrium concentration of (free) phenoxazine as an H-atom donor. Despite their remarkably high reactivity toward peroxyl radicals, the phenoxazines were found to be comparatively stable to one-electron oxidation relative to diphenylamines and phenothiazines (<i>E</i>° ranging from 0.59 to 1.38 V vs NHE). Thus, phenoxazines with comparable oxidative stability to commonly used diphenylamine and phenothiazine RTAs had significantly greater reactivity (by up to 2 orders of magnitude). Computations suggest that this remarkable balance in H-atom transfer kinetics and stability to one-electron oxidation results from the ability of the bridging oxygen atom in phenoxazine to serve as both a π-electron donor to stabilize the aminyl radical and σ-electron acceptor to destabilize the aminyl radical cation. Perhaps most excitingly, phenoxazines have “non-classical” RTA activity, where they trap >2 peroxyl radicals each, <i>at ambient temperatures</i>

Initiators Based on Benzaldoximes: Bimolecular and Covalently Bound Systems

Typical bimolecular photoinitiators (PIs) for radical polymerization of acrylates show only poor photoreactivity because of the undesired effect of back electron transfer. To overcome this limitation, PIs consisting of a benzaldoxime ester and various sensitizers based on aromatic ketones were introduced. The core of the photoinduced reactivity was established by laser flash photolysis, photo-CIDNP, and EPR experiments at short time scales. According to these results, the primarily formed iminyl radicals are not particularly active. The polymerization is predominantly initiated by C-centered radicals. Photo-DSC experiments show reactivities comparable to that of classical monomolecular type I PIs like Darocur 1173

The Catalytic Reaction of Nitroxides with Peroxyl Radicals and Its Relevance to Their Cytoprotective Properties

Sterically-hindered nitroxides such as 2,2,6,6-tetramethylpiperidin-<i>N</i>-oxyl (TEMPO) have long been ascribed antioxidant activity that is thought to underlie their chemopreventive and anti-aging properties. However, the most commonly invoked reactions in this contextcombination with an alkyl radical to give a redox inactive alkoxyamine or catalysis of superoxide dismutationare unlikely to be relevant under (most) physiological conditions. Herein, we characterize the kinetics and mechanisms of the reactions of TEMPO, as well as an <i>N</i>-arylnitroxide and an <i>N</i>,<i>N</i>-diarylnitroxide, with alkylperoxyl radicals, the propagating species in lipid peroxidation. In each of aqueous solution and lipid bilayers, they are found to be significantly more reactive than Vitamin E, Nature’s premier radical-trapping antioxidant (RTA). Inhibited autoxidations of THF in aqueous buffers reveal that nitroxides reduce peroxyl radicals by electron transfer with rate constants (<i>k</i> ≈ 10⁶ to >10⁷ M^–1 s^–1) that correlate with the standard potentials of the nitroxides (<i>E</i>° ≈ 0.75–0.95 V vs NHE) and that this activity is catalytic in nitroxide. Regeneration of the nitroxide occurs by a two-step process involving hydride transfer from the substrate to the nitroxide-derived oxoammonium ion followed by H-atom transfer from the resultant hydroxylamine to a peroxyl radical. This reactivity extends from aqueous solution to phosphatidylcholine liposomes, where added NADPH can be used as a hydride donor to promote nitroxide recycling, as well as to cell culture, where the nitroxides are shown to be potent inhibitors of lipid peroxidation-associated cell death (ferroptosis). These insights have enabled the identification of the most potent nitroxide RTA and anti-ferroptotic agent yet described: phenoxazine-<i>N</i>-oxyl

Gene expression of (A) and (B) in achenes (black columns) and the receptacle (open columns) of small-sized green (G) and red (R) fruits

Transcript levels were analysed by qPCR as described in Materials and methods. Gene expression is shown as relative expression normalized with receptacle tissue from small-sized green fruit (G). Hormonal control of (C) and (D) gene expression. The achenes were carefully removed at mid-sized green stage and the fruits were harvested after 5 d. Additionally, deachened green fruits were treated with a lanolin paste containing the synthetic auxin NAA. Gene expression was analysed by qPCR as described in Materials and methods and the data were normalized against untreated strawberries with the achenes still attached to the fruit.<p><b>Copyright information:</b></p><p>Taken from "Multi-substrate flavonol -glucosyltransferases from strawberry () achene and receptacle"</p><p></p><p>Journal of Experimental Botany 2008;59(10):2611-2625.</p><p>Published online 17 May 2008</p><p>PMCID:PMC2486459.</p><p></p

Phylogenetic analysis of selected plant secondary product glycosyltransferases

The neighbor-joining tree was calculated with the Treecon software package (). Distance calculation was performed with Poisson correction and insertions/deletions were not taken into account. The tree was rooted using a sterol glycosyltransferase from (AsSGT) as an outgroup. Branch lengths indicate the number of substitutions per site. Bootstrap analysis was performed with 100 replicates and only values above 50% are shown. GenBank accession numbers and sources for the respective protein sequences are: AtUGT73B4, AAD17393 (); AtUGT73B5, AAD17392 (); AtUGT73B2, AAR01231 (); DicGT4, BAD52006 (); DbBet5GT, CAB56231 (); SbF7G, BAA83484 (); Letwi1, CAA59450 (); NtIS10a, AAB36652 (); NtIS5a, AAB36653 (); FaGT7, ABB 92749 (×); AtUGT71C2, AAC35238 (); AtUGT71D1, AAC35239 (); DicGT2, BAD52004 (); DbBet6GT, AAL57240 (); AtUGT71B6, BAB02840 (); FaGT6, ABB92748 (×); FaGT3, AAU09444 (×); NtGT3, BAB88934 (); NtGT1b, BAB60721 (); NtGT1a, BAB60720 (); NtSalGT, AAF61647 (); AtUGT84B1, AAB87119 (); AtUGT84B2, AAB87106 (); CuLimGT, BAA93039 (); FaGT2, AAU09443 (×); FaGT5, ABB92747 (); AtUGT84A3, CAB10327 (); AtUGT84A1, CAB10326 (); AtUGT84A2, BAB02351 (); BnSinGT, AAF98390 (); FaGT4, AAU09445 (×); PhA3RhaT, CAA81057 (); Ip3GGT, BAD95882 (); In3GGT, BAD95881 (); GtF3GT, BAA12737 (); DicGT3, BAD52005 (); VmUFGT1, BAA36972 (); PhF3GalT, AAD55985 (); DicGT1, BAD52003 (); VvGT1, AAB81682 (); FaGT1, AAU09442 (×); AtUGT78D1, NP_564357 (); AtUGT78D2, NP_197207 (); AsSGT, CAB06081 ().<p><b>Copyright information:</b></p><p>Taken from "Multi-substrate flavonol -glucosyltransferases from strawberry () achene and receptacle"</p><p></p><p>Journal of Experimental Botany 2008;59(10):2611-2625.</p><p>Published online 17 May 2008</p><p>PMCID:PMC2486459.</p><p></p

Sequence alignment of quercetin and kaempferol -glucosyltransferases, including two already crystallized proteins from (VvGT1) and (UGT71G1) as well as × GT6 (FaGT6), and × GT7 (FaGT7)

The alignment was performed using ClustalX (). Conserved amino acids are shaded, amino acids within 5 Å to kaempferol in the protein model of VvGT are boxed.<p><b>Copyright information:</b></p><p>Taken from "Multi-substrate flavonol -glucosyltransferases from strawberry () achene and receptacle"</p><p></p><p>Journal of Experimental Botany 2008;59(10):2611-2625.</p><p>Published online 17 May 2008</p><p>PMCID:PMC2486459.</p><p></p

Photoinitiators with β-Phenylogous Cleavage: An Evaluation of Reaction Mechanisms and Performance

Bimolecular photoinitiators based on benzophenone and <i>N</i>-phenylglycine ideally overcome limitations of classical two-component systems, such as the possibility of deactivation by a back electron transfer or the solvent cage effect. Furthermore, if they are covalently linked, loss of reactivity by diffusion limitation could be reduced. Here we show that such an initiator displays unusually high photoreactivity. This is established by photo-DSC experiments and mechanistic investigations based on laser flash photolysis, TR-EPR, and photo-CIDNP. The β-phenylogous scission of the C–N bond is highly efficient and leads to the production of reactive initiating radicals at a short time scale